1. Field of the Invention
The present invention relates to exchangeable lens assemblies for use with an optical apparatus such as a camera body or a projector body including an automatic focusing device and more particularly it relates to such exchangeable lens assemblies selectively adjustable for focusing either by manual operation or by operation of the automatic focusing device of the optical apparatus.
2. Description of Prior Art
Japanese patent laid open publication No. Sho. 50-151144 published on Dec. 4, 1975 discloses a camera focusing system which enables both manual focus adjustment and automatic focus adjustment. The system includes an automatic focusing device having a driving shaft formed with a gear, and a lens assembly having a focus adjusting mechanism operable for focus adjustment of an optical system and a manual focusing ring axially shiftable between two predetermined positions. With the manual focusing ring being at either one of the two positions, either the manual focusing ring or the driving shaft is selectively coupled to the focus adjusting mechanism to the exclusion of the other, enabling smooth manual or automatic focusing operation. However, the Japanese patent laid-open publication fails to disclose how the system can apply to an exchangeable lens assembly detachably mountable on a camera body which is to include the automatic focusing device. For example, it lacks concrete disclosure of the manner of disconnecting the driving shaft from a drive motor of the automatic focusing device if the lens assembly is separated from the camera.
U.S. Pat. No. 4,309,077 discloses a lens assembly of the exchangeable type which is selectively adjustable either by manual operation or by an operation of an automatic focusing device included in a camera body on which the lens assembly is detachably mountable. With the lens assembly having been mounted on the camera body, the automatic focusing device is always coupled to a focus adjusting mechanism of the lens assembly whereas a manual focusing ring is coupled to the focus adjusting mechanism only when it is shifted to a particular axial position and then rotated to a particular angular position. Accordingly, the operation required for the changeover from automatic focus adjustment to manual focus adjustment is troublesome.
Moreover, in the case of the manual focus adjustment, the automatic focusing device of the camera body unnecessarily resists against rotation of the manual focusing ring, giving unfavorable operational feeling to a camera operator, because the coupling between the automatic focusing device and the focus adjusting mechanism is always maintained once the lens assembly is mounted on the camera body. The coupling between the automatic focusing device and the focus adjusting mechanism in the case of the manual adjustment also causes a drive motor included in the automatic focusing device to be forcibly rotated so that an undesirable electromotive force is generated. Additionally, a gear transmission included in the automatic focusing device for transmitting the motor rotation to the focus adjusting mechanism generates unpleasant sounds due to engagements of gears being rotated, and furthermore freedom of design is restricted because a reduction gear device comprising of a worm gear and a worm wheel, which allows only one way drive transmission, cannot be used in the gear transmission.
U.S. Pat. No. 4,274,174 discloses a lens assembly of the exchangeable type which includes a pair of focus adjusting mechanisms, one being coupled to a manual focusing ring for manual focus adjustment and the other being capable of been coupled to an automatic focusing device of a camera body for automatic focus adjustment when the lens assembly is mounted on the camera body. In the embodiments disclosed, the lens assembly is provided with a change-over member separately from a manual focusing ring for change-over between manual focus adjustment and automatic focus adjustment and the change-over to the automatic focus adjustment by the operation of the change-over member is made possible with the manual focus adjusting ring being rotated to a particular position as similar to the case of the disclosure of U.S. Pat. No.4,309,077. Additionally, the construction of the lens assembly is complicated and costly because of the inclusion of the two focus adjusting mechanisms.
A lens assembly of the exchangeable type for use with a camera body having an automatic focusing device has been also proposed by copending Miki et al. U.S. patent application Ser. No. 392,326 filed on June 25, 1982 , now U.S. Pat. No. 4,413,894 and co-assigned to the same assignee. With the lens assembly proposed in this patent, a single focus adjusting mechanism is provided for both manual focus adjustment and automatic focus adjustment, and, when a manual focusing ring is shifted to a particular axial position for automatic focus adjustment, a driven shaft coupled with the focus adjusting mechanism is shifted to an engaging position to engage a driving shaft of the automatic focusing device of the camera body and at the same time the manual focusing ring is uncoupled from the focus adjusting mechanism. In contrast, when the manual focusing ring is shifted to another particular axial position for manual focus adjustment, the driven shaft is shifted to a disengaging position to disengage from the driving shaft while the manual focusing ring is coupled with the focus adjusting mechanism. Accordingly the lens assembly is relatively simple in construction and free from the disadvantages caused by coupling of the automatic focusing device with the focus adjusting mechanism even in the case of the manual focus adjustment in U.S. Pat. No. 4,309,077. However, as it is constructed such that the coupling between the manual focusing ring and the focus adjusting mechanism in the case of the manual focus adjustment is achieved by engagement of a pair of rigid gears one of which is shifted relative to the other in the direction parallel to the optical axis, there occurs a case where the gears cannot engage one another due to failure of alignment of their gear teeth. In such a condition rotation of the manual focusing ring cannot be transmitted to the focus adjusting mechanism, resulting in failure of the manual focus adjustment.
Meanwhile the above prior art lens assemblies except that disclosed in U.S. Pat. No. 4,272,174 are common in that the change-over between the manual focus adjustment and the automatic focus adjustment is made by axial operation of a manual focusing ring which is rotated for manual focus adjustment. This is advantageous in that the change-over operation and the manual focusing operation are made without separating camera user's fingers from the manual focusing ring but disadvantageous in that, if the manual focusing ring is axially shifted by application of an axial force during the manual focusing operation, there occurs a failure of the manual focus adjustment because of uncoupling of the manual focusing ring from the focus adjusting mechanism. To avoid such a failure, the coupling between the manual focusing ring and the focus adjusting mechanism must be long enough in the axial direction and a large distance must be provided between the axial position of the manual focusing ring for manual focus adjustment and that for automatic focus adjustment. Additionally, for example in the case of the lens assembly disclosed in the aforementioned U.S. Pat. No. 4,413,894 there is a disadvantage that indication of the camera-to-object distance to which the optical system is adjusted cannot be made by use of a distance scale and an index therefor because the manual focusing ring is coupled with the focus adjusting mechanism at any angular position.